1. Field of the Invention
This invention relates to a magnetoresistive-effect element comprising a magnetoresistive-effect thin film, a pair of hard magnetic layers formed respectively at opposite ends of the magnetoresistive-effect thin film and electrode layers formed respectively at the sides of the oppositely disposed main surfaces of the magnetoresistive-effect thin film.
2. Related Background Art
Magnetoresistive-effect magnetic heads (to be referred to as MR head hereinafter) adapted to read information signals recorded on a magnetic recording medium, utilizing the magnetoresistive-effect of magnetoresistive-effect thin film (to be referred to as MR thin film hereinafter), are widely used in many high density magnetic recording/reproduction apparatus including hard disk apparatus and magnetic tape apparatus.
Of MR heads, so called shield type MR heads comprising a magnetoresistive-effect element (to be referred to as MR element hereinafter) sandwiched by a pair of magnetic shield member have been finding practical applications.
Various different types of MR thin film are known to date, including one showing the anisotropic magnetoresistive-effect (AMR), one showing the giant magnetoresistive-effect (GMR) and one showing the tunnelling magnetoresistive-effect (TMR).
Of such known various types of MR thin films, MR thin film that utilizes the tunnelling magnetoresistive-effect (to be referred to as TMR thin film hereinafter) comprises a magnetization fixing layer made of an anti-ferromagnetic material, a pin layer made of a ferromagnetic material a tunnel barrier layer made of a nonmagnetic nonconductive material and a free layer made of a ferromagnetic material, said layers being laid sequentially one on the other.
When a sense current is made to flow substantially perpendicularly relative to the TMR thin film, a tunnelling current flows in the tunnel barrier layer and the flow is directed toward from one of the ferromagnetic layer to the other ferromagnetic layer. This phenomenon is referred to as tunnelling junction type magnetoresistive-effect. In an MR element that utilizes TMR thin film (to be referred to as TMR element hereinafter), the magnetization of the free layer changes as a function of the external magnetic field to consequently change the conductance of the tunnelling current. The external magnetic field is detected by observing the conductance of the tunnelling current.
The change in the conductance-of the tunnelling-current is dependent on the relative angle of the two ferromagnetic layers as viewed in the direction of magnetization. In the case of TMR thin film, theoretically the magnetic reluctance ratio of the two ferromagnetic layers can be determined from their respective polarizabilities of magnetization. Thus, TMR elements are attracting attention from the viewpoint of using them for MR heads.
So-called spin valve film is a type of MR thin film utilizing the giant magnetoresistive-effect (to be referred to as GMR thin film hereinafter) and comprises a magnetization fixing layer made of an anti-ferromagnetic material, a pin layer made of a ferromagnetic material, an intermediate layer made of a nonmagnetic nonconductive material and a free layer made of a ferromagnetic material, said layers being laid sequentially one on the other.
When an external magnetic field is applied to a GMR element, the magnetization of the free layer is defined as a function of the direction and the intensity of the applied external magnetic field. The electric resistance of the spin valve layer is maximized when the direction of magnetization of the pin layer and that of the free layer are differentiated from each other by 180xc2x0 and minimized when they are made same relative to each other. Therefore, the pin valve film changes its electric resistance as a function of the external magnetic field applied to it. Thus, the external magnetic field can be detected by observing the change in the electric resistance.
Meanwhile, for the MR head, it is important to control the magnetic domains and make the free layer of the MR thin film have a single magnetic domain in order to suppress the Barkhausen noise.
An MR head 100 utilizing the anisotropic magnetoresistive-effect or the giant magnetoresistive-effect comprises a lower magnetic shield layer 102a and a lower gap layer 103a laid sequentially on a substrate 101. Then, an MR thin film 104 is formed on the lower gap layer 103a and a pair of bias layer 105, 105 are formed respectively at opposite ends of the MR thin film 104. An upper gap layer 103b and an upper magnetic shield layer 102b are formed on the MR thin film 104 and the bias layers 105, 105.
The free layer of the MR thin film 104 is made to have a single magnetic domain by arranging a pair of bias layers 105, 105 to be used to apply a bias magnetic field to the MR thin film 104 respectively at opposite ends of the MR thin film 104. The bias layers 105, 105 are made of a hard magnetic material that is electrically conductive such as CoPt.
An MR head using a TMR thin film (to be referred to as TMR head hereinafter) comprises a lower magnetic shield layer, a lower gap layer, a TMR thin film, an upper gap layer, an upper magnetic shield layer that are laid sequentially on a substrate. The lower magnetic shield layer, the lower gap layer, the upper gap layer and the upper magnetic shield layer are designed to operate as electrodes.
Then, a sense current is made to flow substantially perpendicularly relative to the film surface of the TMR thin film and the conductance of the tunnelling current that flows through the tunnel barrier layer of the TMR thin film is observed to read the magnetic signal applied to it.
The above described bias layers 105, 105 are made of a hard magnetic material that is electrically conductive such as Co.Pt. Therefore, when bias layers are arranged respectively at opposite ends of the TMR head, the sense current can be diverted into the bias layers to make it difficult to read the magnetic signal applied to it. Because of this reason, in the case of a TMR head, it is not appropriate to control magnetic domains by arranging bias layers respectively at opposite ends of the MR thin film.
In recent years, a GMR head adapted to a so-called CPP (current perpendicular to the planexe2x80x94to be referred to as CPP-GMR head hereinafter) and formed by arranging a gap layer and a shield layer that are designed to operate as electrode layer as shown in FIGS. 2 and 3 has been proposed as MR head using GMR thin film.
A CPP-GMR head comprises a lower magnetic shield layer, a lower gap layer, a GMR thin film, an upper gap layer and an upper magnetic shield layer that are laid sequentially on a substrate, of which the lower magnetic shield layer, the lower gap layer, the upper gap layer and the upper magnetic shield layer are designed to operate as electrode layers.
Then, a sense current is made to flow substantially perpendicularly relative to the film surface of the GMR thin film and the conductance of the electric current that flows through the intermediate layer of the GMR thin film is observed to read the magnetic signal applied to it.
As pointed out above, a CPP-GMR head is so designed as to make a sense current flow perpendicularly relative to the GMR thin film. Then, the rate of change of the electric current in the CPP-GMR head is larger when it is made to flow perpendicularly relative to the GMR thin film than when it is made to flow in parallel with the GMR thin film. Additionally, since the electrode layers are made to operate as shield layers, it is no longer necessary to electrically insulate the electrode layers and the shield layers if the gap is made narrow and the manufacturing process can be simplified. For theses reasons, CPP-GMR beads have been attracting attention and are getting popularity as magnetic heads.
However, as in the case of a TMR head, when bias layers are arranged respectively at opposite ends of the GMR head, the sense current can be diverted into the bias layers to make it difficult to read the magnetic signal applied to it. Because of this reason, in the case of a CPP-GMR head too, it is not appropriate to control magnetic domains by arranging electrically highly conductive bias layers respectively at opposite ends of the MR thin film.
To solve the above identified problem, a technique of arranging bias layers after forming insulation layers respectively at opposite ends of the MR thin film of a TMR head or a CPP-GMR head has been used. With this arrangement, however, it is no longer possible to apply a sufficiently strong bias magnetic field to the MR thin film to make it difficult to control the magnetic domains of the free layer.
There is also known a technique of arranging bias layers in such a way that they contact only the free layer of the MR thin film. However, this technique is accompanied by the problem as will be discussed hereinafter. Now, a CPP-GMR head where bias layers are arranged so as to contact only the free layer of the MR thin film and the problem that arises from such a head will be discussed.
FIG. 2 schematically illustrates the structure of a CPP-GMR head 111 prepared by using a so-called bottom type GMR thin film 110 where a pin layer is formed at the side of the lower magnetic shield layer and a free layer is formed thereon and also by applying the above technique.
Referring to FIG. 2, the CPP-GMR head 111 comprises a lower magnetic shield layer 113a and a lower gap layer 114a laid sequentially on a substrate 112. Then, a magnetization fixing layer 115, a pin layer 116 and an intermediate layer 117 are formed on the lower gap layer 114a and a pair of nonmagnetic layers 118, 118 are formed respectively at opposite ends of these layers. The nonmagnetic layers 118, 118 are made substantially flush with the intermediate layer 117. Then, a free layer 119 and a protection layer 120 are laid sequentially on the intermediate layer 117 and upwardly tapered. A pair of bias layers 121, 121 are formed respectively at opposite ends of these layers. Then, an upper gap layer 114b and an upper magnetic shield layer 113b are laid sequentially on the protection layer 120 and the bias layers 121, 121.
FIG. 3 schematically illustrates the structure of a CPP-GMR head 131 prepared by using a so-called top type GMR thin film 130 where a free layer is formed at the side of the lower magnetic shield layer and a pin layer is formed thereon and also by applying the above technique.
Referring to FIG. 3, the CPP-GMR head 131 comprises a lower magnetic shield layer 133a and a lower gap layer 134a laid sequentially on a substrate 132. Then, a backing layer 135 and a free layer 136 are laid sequentially on the lower gap layer 134a. A pair of bias layers 137, 137 are formed respectively at opposite ends of these layers. The free layer 136 is made substantially flush with the bias layers 137, 137. Then, an intermediate layer 138, a pin layer 139, a magnetization fixing layer 140 and a protection layer 141 are laid sequentially on the free layer 136. A pair of nonmagnetic layers 142, 142 are formed respectively at opposite ends of theses layers. Then, an upper gap layer 134b and an upper magnetic shield layer 133b are laid sequentially on the protection layer 141 and the nonmagnetic layers 142, 142.
However, with the above arrangement again, the sense current can be diverted into the bias layers 121, 121. Additionally, the productivity is low because of the complicated manufacturing process.
In view of the above identified circumstances of the prior art, it is therefore the object of the present invention to provide a magnetoresistive-effect element comprising a pair of hard magnetic layers formed respectively at opposite ends of the magnetoresistive-effect thin film that can reduce the electric current diverted into the hard magnetic layers if electrode layers are formed oppositely on the main surfaces of the magnetoresistive-effect thin film and operates reliably for signal reproduction.
According to the invention, the above object is achieved by providing a magnetoresistive-effect element comprising a magnetoresistive-effect thin film operating as magnetism sensing section, a pair of hard magnetic layers formed respectively at opposite ends of said magnetoresistive-effect thin film and adapted to apply a bias magnetic field to said magnetoresistive-effect thin film and a pair of electrode layers formed oppositely on the main surfaces of the magnetoresistive-effect thin film and adapted to supply an electric current to said magnetoresistive-effect thin film, said hard magnetic layers having an electric resistivity not lower than 0.5 xcexa9cm.
A magnetoresistive-effect element according to the invention and having the above described configuration allows little electric current to divert into the hard magnetic layers and makes is possible for the hard magnetic layers to apply a bias magnetic field to the magnetoresistive-effect thin film if a pair of hard magnetic layers are formed respectively at opposite ends of the magnetoresistive-effect thin film and a pair of electrode layers are formed oppositely on the main surfaces of the magnetoresistive-effect thin film.
As pointed out above, a magnetoresistive-effect element according to the invention comprises a pair of hard magnetic layers that are made of a material showing a high electric resistivity. Therefore, if an electric current is made to flow perpendicularly relative to the surface of the magnetoresistive-effect thin film, it is prevented from diverting into the hard magnetic layers.
Additionally, in a magnetoresistive-effect-element according to the invention, a pair of hard magnetic layers are formed directly and respectively at opposite ends of the magnetoresistive-effect thin film. Therefore, it is possible to apply a sufficiently strong bias magnetic field to the first ferromagnetic layer to make it show a single magnetic domain so that any magnetic wall can hardly appear in the element. As a result, the Barkhausen noise can hardly occur and it is possible to detect any weak external magnetic field.